MUHANGUZI et al. 91 Tropical Ecology 46(1): 91-98, 2005 ISSN 0564-3295 © International Society for Tropical Ecology Forest site disturbances and seedling emergence in Kalinzu Forest, Uganda HOSEA D.R MUHANGUZI1, JOSEPH OBUA2∗, HANNINGTON OREYM-ORIGA3 & OLE R. VETAAS4 1Department of Science & Technical Education, Makerere University P.O. Box 7062, Kampala, Uganda; 2 Department of Forest Biology & Ecosystems Management, Makerere University P.O Box 7062 Kampala, Uganda; 3Department of Botany, Makerere University, University P.O. Box 7062, Kampala, Uganda; 4Centre for Development Studies, Bergen University, Stomgt. 54, N-5007, Bergen, Norway Abstract: The influence of forest site manipulations (loosening soil, weeding and removing topsoil) and forest light environment on emergence of seedlings of selected pioneer, canopy and understorey tree species were assessed in Kalinzu Forest in areas with different management histories. The species were Craterispermum laurinum, Funtumia africana, Musanga leo- errerae, Parinari excelsa, Strombosia scheffleri and Trema orientalis. Most seedlings (> 38 %) emerged where the soil was loosened and least (< 10 %) in the undisturbed plots. With the exception of the pioneer species (T. orientalis and M. orientalis), seedlings of other species emerged in all the manipulated plots. Seedlings of the pioneer species only emerged in plots with light intensity ≥90 % while those of the canopy and sub-canopy species (P. excelsa, F. africana and S. scheffleri) emerged in plots where light intensity ranged from 6 % to 100 %. Resumen: Se evaluó la influencia de las manipulaciones locales del bosque (aflojamiento del suelo, desyerbe y remoción del suelo superficial) y del ambiente lumínico del bosque sobre la emergencia de plántulas de una selección de especies arbóreas pioneras, de dosel y de sotobosque en el Bosque Kalinzu, en áreas con diferentes historias de manejo. Las especies fueron Musanga leo-errerae, Trema orientalis, Funtumia africana, Strombosia scheffleri, Parinari excelsa y Craterispermum laurinum. La emergencia de plántulas alcanzó su máximo (> 38%) donde el suelo fue aflojado y su mínimo (< 10 %) en las parcelas sin disturbio. A excepción de las especies pioneras (T. orientalis y M. orientalis), las plántulas de otras especies emergieron en todas las parcelas manipuladas. Las plántulas de las especies pioneras sólo emergieron en parcelas con una intensidad luminosa ≥ 90%, mientras que las de especies del dosel y del subdosel (P. excelsa, F. africana y S. scheffleri) emergieron en parcelas donde la intensidad de la luz fluctuó de 6% a 100%. Resumo: A influência das manipulações da estação florestal (revolvimento do solo, monda de ervas e remoção da camada superior do solo) e o ambiente luminoso da floresta na emergência das plântulas de pioneiras seleccionadas, o copado e as espécies de árvores no sub bosque foram determinadas em áreas com diferentes histórias de manejo na Floresta de Kalinzu. As espécies foram a Musanga leo-errerae, Trema orientalis, Funtumia africana, Strombosia scheffleri, Parinari excelsa e Craterispermum laurinum. A maior parte das plântulas (>38%) emergiram quando o solo foi revolto e foi menor (<10%) nas parcelas não disturbadas. Com excepção das espécies pioneiras (T. orientalis e M. orientalis), as plântulas das outras espécies emergiram em todas as parcelas manipuladas. As plântulas das espécies pioneiras só emergiram nas parcelas com intensidade luminosa ≥ 90% enquanto que as do ______________________________________________________________________________________________ ∗ Corresponding Author 92 FOREST DISTURBANCES IN UGANDA copado e sub-copado (P. excelsa, F. africana e S. scheffleri) emergiram nas parcelas em que a intensidade luminosa se situou entre os 6% e os 100%. Key words: Canopy, germination, pioneer species, seedling emergence, site manipulation, tropical forest. Introduction tolerant species can establish in either gaps or understorey (Forget 1991). Seed germination, emergence, growth, survival Whereas forest site disturbances can result and establishment of seedlings affect forest from natural tree falls, logging is a major cause of regeneration by influencing plant populations in forest disturbances (Whitmore 1992). This study tropical forests (Osunkjoya et al. 1992). These was carried out in Kalinzu forest reserve in areas processes vary with species and forest habitat that experienced different logging levels. The characteristics (Clark 1990; Pons 1992; Schupp objectives were to determine (1) the effect of 1988; Tripathi & Khan 1990). Habitat different plot manipulations (loosening the topsoil, characteristics such as degree of canopy openness weeding out herbs and shrubs and removing and nature of soil can be affected by various kinds topsoil) on seedling emergence, (2) the species of forest disturbances (Boot & Gullison 1995; composition and abundance of the tree seeds Denslow 1995). present in the forest soil seed bank and, (3) the Soil disturbance can influence germination and effect of different forest light environments (under seedling emergence because it exposes some of the closed canopy, forest gap edge and gap centre) on formerly buried seeds to incident light, which may seedling emergence. In this article, the term induce some of them to germinate (Denslow 1980; ‘emerged seedling’ refers to those very young Jensen 1995; Pugnaire & Lozano 1997). For seedlings that have just emerged above the ground example, Putz & Appanah (1987) noted that surface. pioneer tree species are most abundant in open forest areas where soil has been disturbed. Materials and methods Disturbance that loosens soil may enhance seed germination and seedling emergence because of Study area and location of study plots improved aeration and temperature variation This study was carried out in Kalinzu Forest (Clark & Clark 1987; Denslow 1987). Reserve, which is located in southwestern Uganda Forest soil disturbance also affects the soil seed between 00 17' S - 00 30' S and 300 00' - 300 07' E and bank, which often contains dormant seeds (Riswan has an altitudinal range of 1250 - 1827 m above sea & Kartawinta 1991; Silvertown 1987). While seeds level. It experiences two rainfall peaks from March of many tropical trees have short longevity to May and September to November. The annual (Vasquez-Yanes & Orozico-Segovia 1996), seeds of rainfall ranges between 1,150 - 1400 mm (Howard some species, especially the pioneer species often lie 1991). Minimum and maximum temperature dormant in the soil seed bank (Ellison et al. 1993; ranges are 140 - 170 C and 250 - 280 C, respectively. Thompson 1992). Soil disturbance can expose According to Howard (1991), this forest can be formerly buried and dormant seeds to the ground classified as a medium altitude moist evergreen surface consequently promoting their germination forest and medium altitude moist semi-deciduous (Everham et al. 1996). forest. Two hundred and sixty five tree species Augspurger (1984) noted that light strongly (more than 57 percent of the country's total) have influences regeneration of some tropical tree been recorded in this forest (Howard 1991). species. According to Martinez-Ramos & Soto- Study plots were located in three areas of the Castro (1995), pioneer species and secondary forest forest having different management histories species usually germinate and establish in gaps. (Howard 1991). One area was heavily On the other hand, seedlings of many shade MUHANGUZI et al. 93 mechanically logged between 1950 and 1975 and is forest and forest edge coloniser) and referred to as forest type A (FTA). The second area Craterispermum laurinum (Poir.) Benth. (an was heavily pitsawn while the third was understory species, Hamilton 1991) were sown in minimally disturbed, and these areas are referred the weeded and undisturbed plots. Other seeds to as forest types B (FTB) and C (FTC), were sown in plots where the soil was loosened and respectively. In each of the three forest types, a 1 x the topsoil removed. The location of each seed was 1650 m transect was established. Thirty-three mapped and ten replicates made for each type of study plots, each 20 x 20 m in size, were established manipulation. Before sowing the seeds, their in each forest type and laid out in an alternating viability was tested using a technique suggested fashion at 30 m intervals along each transect. by Burke (1970), whereby tetrazolium solution is Seedling emergence was monitored in sub applied to freshly exposed cotyledons of sliced plots established within each of the big plots of 20 seeds. Fresh and air-dried seeds were used x 20 m size. The sub-plots measuring 2 x 2 m were because older seeds tend to be sensitive to light located at a distance of one metre apart. In each quality (Vasquez-Yanes & Orozco-Segovia 1996). forest type, 33 plots were manipulated in a Fifty seeds of M. leo-errerae, T. orientalis, F. particular manner e.g. some plots were africana and C. laurinum were sown in the manipulated by weeding out existing herbs and replicate plots while only 30 and 20 seeds of S. shrubs while others had their topsoil removed to a scheffleri and P. excelsa, respectively, were sown depth of about 10 cm so as to remove seeds already because they had few fresh and viable seeds at the present in the soil prior to this study. Other plots time. All seedlings that emerged were enumerated. were manipulated by loosening the topsoil and removing roots and any other plant remains. Some Soil seed bank analysis plots were left undisturbed while others were